1. Field of the Invention
The present invention is in the field of amplifiers, and relates particularly to amplifiers which drive loads such as speakers that are highly reactive and are also subject to mechanical distortion influences including inertia and resonances.
2. Description of the Prior Art
For more than forty years, and still according to the current state of the art, audio amplifiers have employed what is commonly referred to as "voltage feedback" in an endeavor to improve frequency response and reduce distortion. Such voltage feedback systems are sometimes referred to in the art as "constant voltage" systems, since for a fixed amplifier input voltage the output voltage remains substantially constant over a broad frequency range or bandwidth. Thus, present audio amplifiers are capable of providing a voltage output for driving a speaker which quite accurately follows the amplifier input program voltage, as to both waveform or shape and phase.
However, the conventional magnetic coil-driven speaker (or multiple speaker system) used as a load for such amplifier is essentially a current-driven device, and it has both electrical characteristics and mechanical characteristics which seriously alter the flow of current through it independently of "constant voltage" amplifier drive, thereby preventing the speaker current from coming even close to following the voltage output of the amplifier and instead causing the speaker to depart considerably from the program applied by the amplifier in amplitude, waveform and phase. The general result is that the acoustic response of the speaker is considerably different than the flat voltage response of the "constant voltage" amplifier.
The conventional coil-driven speaker (or multiple speaker system) is a load that is highly inductively reactive. This causes the load impedance to vary with frequency, becoming higher, with power to the speaker consequently lower, as program frequencies rise above the usual 400 Hz nominal or rated impedance point all of the way out to 20 KHz and higher. By way of example, to illustrate how serious this increase in impedance at higher frequencies can be, the manufacturer's impedance/frequency response curve for a typical state-of-the-art speaker rated 8 ohms at 400 Hz shows the impedance to be doubled to 16 ohms at approxinmately 4300 Hz, and quadrupled to 32 ohms at about 10 KHz. Power is inversely proportional to impedance in a speaker.
The inductive reactance of the speaker load also causes load current to lag in phase from the program, and this phase lag increases with frequency from the 400 Hz rated impedance point all the way to the high end of the sound frequency spectrum. By way of example to show how serious this inductive reactance phase lag can be, measurements made with the speaker referred to in the immediately preceding paragraph showed a phase lag of approximately 33.degree. at 400 Hz, approximately 45.degree. at 900 Hz, and approximately 70.degree. at 5 KHz. As an integral part of this inductive reactance phase lag, the rise times for high frequency wave fronts or transients which usually contain important program information are slowed way down relative to the actual wave fronts or transients in the program.
The mass of a speaker resists acceleration and deceleration in response to respective rising and falling wave fronts, resulting in inertial lag and overshoot, respectively. Inertial distortion of speaker acoustic output is surprisingly close or analogous to the phase and rise time distortions produced by the inductive reactance of a speaker, so that these effects of inductive reactance and inertia are additive. The adverse effects of inertia, like those of inductive reactance, increase with increasing frequency all of the way out to the high end of the audible spectrum.
Most sounds that are produced by musical instruments have a sharp attack that is characterized by a sharply rising initial transient wave front in each fundamental frequency cycle, this initial transient containing most of the high frequency harmonic content of the sound. It has been found that for the human ear to hear the entire spectrum of such sounds, it must receive these initial high frequency harmonic sounds first, followed then by the midrange and low end frequencies. However, the additive or cumulative effects of speaker inductive reactance and inertia in current state-of-the-art amplifier/speaker systems cause the rise time to be so slow and the phase lag to be so large at higher frequencies that the sharply rising wave fronts or initial transients which contain most of the high frequency harmonics become masked to a large extent by the lower, heavier frequencies. Such masking of the high frequency harmonics is commonly referred to as "transient distortion", and causes the acoustic output of the system to sound "artificial" or "recorded", instead of sounding completely "live" or "natural" as when the ear properly receives the sharply rising initial transient wave front in its proper order ahead of the lower frequencies.
The typical condenser microphone has a rise time of about 20 to 25 microseconds. In order to make up for the losses at the sound reproduction end, and at the same time reproduce the high frequency harmonic content in the sharply rising initial transient wave fronts captured by such a condenser microphone, it is necessary for an amplifier system to be able to produce a speaker load current rise time that is faster than the microphone rise time, preferably about 10 microseconds or less, and ideally about 5 microseconds or less. Direct solid state pickup transducers such as those manufactured and sold by Barcus-Berry, Inc. of Huntington Beach, Calif., introduce virtually no delay into the program rise time, and where such direct transducers are employed, if the amplifier system has a rise time of not more than about 10 microseconds, and preferably not more than about 5 microseconds, the reproduction will sound completely "live" and "natural". In order to produce such rapid rise times in an amplifier/speaker combination, the speaker load current phase must be held close to "in phase" with the program signal from the 400 Hz nominal impedance point all of the way out to approximately 20 KHz, and preferably the speaker load current should be slightly leading all of the way out to approximately 20 KHz. Additionally, in order for the high frequency harmonics to be heard in their proper proportion relative to the other sound components, the attenuation of these high frequency components resulting from speaker inductive reactance must also be overcome.
Conventional state-of-the art amplifier/speaker systems are not capable of providing speaker load current with the fast rise time and substantially "in phase" condition required to avoid serious masking problems. Various attempts have been made to solve the problem, and while some of these have improved the rise time and others have improved the phasing, none have heretofore simultaneously produced a rise time on the order of 10 microseconds or less and an "in phase" or slightly leading phase condition from about 400 Hz all of the way out to about 20 KHz. A principal type of equipment that is currently employed in attempting to solve this problem is the graphic equalizer. Thus, a quality unit in the hi-fi industry is the Bose equalizer that is marketed with the Bose speaker. This unit has a frequency response curve that is actually too steep and a slightly leading phase up to around 10 KHz, going in phase between about 10 KHz and 15 KHz, but then the response curve falls off rapidly, and there is a large phase lag after 15 KHz, the lag being approximately 45.degree. at 20 KHz. The resulting rise time is about 45 to 50 microseconds, much too slow to reproduce the high frequency harmonics in the initial transient wave front of most musical instrument sounds.
Below the 400 Hz nominal impedance point the impedance curve for a conventional speaker rises in a capacitive reactance effect caused by the compliance and open air cone resonance of the speaker. The manufacturer's speaker response curve referred to above shows the impedance to rise sharply below about 150 Hz up to a large impedance peak at 50 Hz, and then to slope back down sharply to 8 ohms at 20 Hz. This large low frequency impedance peak represents a big hole in the acoustic output of the speaker, so that much of the low frequency information in the program is lost. While some cabinet designs are effective to reduce open air cone resonance at low frequencies, they generally introduce further problems such as speaker cabinet resonances and undesired damping. Unless amplifier compensation can be provided to raise up the low frequency response all of the way down to about 20 Hz, much of the low frequency content will be lost, and particularly the low frequency information representing the percussion sounds.
In addition to the poor low frequency response of the typical speaker, the capacitive reactance effect in the region of the rising slope of the cone resonance part of the response curve below 400 Hz produces a seriously leading phase, causing fundamental and low harmonic frequencies in this region to, in effect, overtake the simultaneously phase-lagging high frequency components of the program, further compounding the "masking" problem referred to above.
The most pertinent prior art of which applicant is aware is the Crooks U.S. Pat. No. 4,260,954, issued Apr. 7, 1981 for "Amplifier Load Correction System". The load correction system of this prior patent developed feedback voltage signal representative of current through the driven load, made a comparison between such feedback signal and the amplifier input program voltage, and utilized the results of this comparison to adjust the gain of the program amplifier line to compensate for load current deviations in waveform and phase from the program. This prior system worked well when it sensed the load current of a single speaker, but attempts to employ it in connection with a load that was more complex than a single speaker, such as a plurality of speakers with crossover networks or a multiple speaker distribution line system, were not always satisfactory because the more complex load currents did not necessarily truly represent speaker performance, and the sensing could thus be masked. Another problem with direct sensing of the real load as in Crooks U.S. Pat. No. 4,260,954 was that the real load was unpredictable and often imperfect, having undesirable impedance humps and dips relative to frequency. The prior system was not capable of straightening out such irregularities. The system of U.S. Pat. No. 4,260,954 also had the difficulty that it could not be simply plugged into the amplification line; it also required additional connections to the driven load.
West German patent No. 2,235,664, published Jan. 31, 1974, was one of the references cited in said Crooks U.S. Pat. No. 4,260,954, the FIG. 2 circuit of that reference being relied upon. However, such circuit as shown and described in this German patent does not suggest use of a reference or model load to correct for the adverse effects of inductive reactance, inertia and resonances of the real speaker load. In fact, the circuit of the German patent does not in any way tend to correct for deviations of speaker load current or acoustic output from the incoming program.
Other references relied upon in said U.S. Pat. No. 4,260,954, which may be of interest relative to the present invention because of a rough general similarity in circuit appearance to circuits in one or more of the forms of the present invention are U.S. Pat. No. 3,902,111 issued Aug. 26, 1975 to Pfisterer, Jr., and U.S. Pat. No. 4,153,849 issued May 8, 1979 to Hall et al. However, these two references are considered to constitute non-analogous art, in that they are not dealing with any kind of a source of a program voltage signal that is variable as to waveform, or with the typical amplifier load such as speaker that is intended to be driven in response to such a variable waveform program voltage signal and which conventionally has a load current and power output that varies widely and generally continuously from the program voltage signal in both waveform and phase due to its high reactance, inertia, and resonances. Thus, the circuit shown and described in the Pfisterer, Jr. patent appears to be simply a low frequency servo system, with a fixed set point input and no way to put in a program voltage signal variable as to waveform, and with type and values of circuit components completely unrelated to and leading away from the present invention.
The Hall et al patent relates to a circuit for normalizing the operating characteristics of yttrium-iron-garnet (YIG) devices and traveling-wave maser (TWM) devices that are tuned by a controlled current so that such devices despite variations in individual ones can be interchanged in a microwave system without re-aligning the entire system. Again, no program signal that is variable as to waveform is applicable to the input, but there is simply a reference DC voltage which is applied to shift the response of the device to the required current-frequency characteristic for the pre-aligned overall system. The Hall et al patent discloses a positive feedback circuit arrangement having type and values of circuit components completely inapplicable to and leading away from the present invention.